The present invention relates to image recognition correlators in general, including those using spatial light modulators having phase distortions therein.
Spatial light modulators (SLMs) provide a very attractive way of transforming the traditional two-lens Fourier transform optical processor into a practical real-time system. Several researchers have reported on such systems for pattern recognition by using the systems to compute in analog fashion a correlation integral. See for example the following papers: D. Psaltis, E. Paek, and S. Venkatesh, "Optical Correlation with a Binary Spatial Light Modulator," Opt, Eng. 23, 698 (1984). D. Flannery, A. Biernacki, J. Loomis, and S. Cartwright, "Real-Time Coherent Correlator Using Binary Magnetooptic Spatial Light Modulators at Input and Fourier Planes," Appl. Opt. 25, 466 (1986). See also my U.S. Pat. No. 4,588,260.
It is usually assumed when such an SLM is used as the input means of a coherent optical processor, that the SLM effectively modulates the amplitude a(x,y) of the light exiting the device: EQU .alpha.(x,y)=A.sub.o s(x,y), (1)
where A.sub.o is an arbitrary constant and s(x,y) is the signal input to the first input SLM. It is known that there is an attendant phase shift introduced at each pixel by the SLM-a signal-dependent phase distortion. The liquid crystal light valve shows a linear effect with a slight admixture of quadratic component. However, the exact nature of the phase distortion is unimportant as long as its effect is dealt with in accordance with the present invention.
Initially, it seemed reasonable to others to try and keep this effect small, since one feels instinctively that it could only degrade the correlation process. In accordance with my invention however, this effect is utilized to enhance greatly the correlation response in terms of signal to noise ratio (SNR) and the narrowness of the correlation response, both highly desirable in a practical optical or electronic signal processor.